With rapid development of Internet technology, information exchange using electronic mails is frequently performed. When exchanging electronic mails, reply mails are often generated with quotation from received electronic mails. By exchanging electronic mails many times, many electronic mails exchanged in the past are quoted to make a long electronic mail.
The electronic mail that becomes long by several times of exchange of electronic mails has history information including the past quoted mail messages of several times. If such history information includes secret information, there is a risk that the secret information will be disclosed to an unintended third person by repeatedly transmitting and receiving the new electronic mail with quotation from the received electronic mails.
As a technology for preventing information included in electronic mails from being disclosed to the outside, for example, one conventional approach includes an image transmission system that transmits information on image data of an electronic mail that requests transmission of the image data in encrypted form. Another conventional approach includes an electronic-mail server system in which information on the originator of a return mail is encrypted to allow anonymous originator and recipient to exchange mails. Another conventional approach includes an information processing apparatus in which an electronic-mail transmitting section transmits electronic mails in encrypted form. Another conventional approach includes a game-machine management system equipped with a history-information mail box.
However, in some of the conventional approaches, the object to be encrypted is information on image data or an originator, and information itself included in a quoted electronic mail is not encrypted; therefore, a leak of secret information included in the quoted electronic mail cannot be prevented. Also, in some conventional approaches, although a leak of secret information can be prevented because an electronic mail itself is encrypted, not only a quoted electronic mail but also a newly created electronic mail cannot be read. This poses the problem of complicating even a normal mail exchange procedure, thus impairing the usability of the electronic mail system. Also, in some conventional approaches, storing the history information of an electronic mail allows a determination whether secret information is included; however, at the transmission of the electronic mail, transmission of the electronic mail including secret information can be stopped, but the usability of the electronic mail system is inevitably impaired.